A nano-sized tubular aluminum silicate is naturally produced as an imogolite, and an imogolite exists in soil, and is primarily produced in soil deriving from volcanic ash. Moreover, a natural imogolite, together with allophane as the related mineral, affects the transfer of nutrients and moisture in the soil and their supply to plants, and also affects the accumulation and residue of hazardous contaminated substances. The foregoing tubular aluminum silicate has, as its primary constituent elements, silicon (Si), aluminum (Al), oxygen (O) and hydrogen (H), and is a hydrated aluminum silicate assembled from numerous Si—O—Al bonds of a tubular shape having an outer diameter of 2.2 to 2.8 nm, an inner diameter of 0.5 to 1.2 nm, and a length of 10 nm to several μm, and is naturally a clay constituent distributed in the soil having as its base material ash fall ejecta such as volcanic ash and pumice.
A protoimogolite is a substance that is known to be a precursor of the imogolite, and possesses the property that derives from the imogolite structure. Consequently, an imogolite and a protoimogolite have roughly the same adsorption behavior at a relative humidity of 20% or less even regarding water vapor adsorption characteristics, and, although the protoimogolite does grow to a relatively long tube shape as with a crystalline imogolite, it is considered to possess the imogolite structure. Accordingly, a protoimogolite possesses a similar adsorbent property as with an imogolite in a low-humidity range.
The unique shape and physical properties of an imogolite as the nano-sized tubular aluminum silicate are considered to be industrially useful. In other words, since an imogolite is characterized in being able to adsorb various substances based on its unique microstructure, for instance, it has been conventionally said that an imogolite has application potency as a hazardous contaminant adsorbent, a deodorant, or a gas storage material of carbon dioxide, methane and the like. In addition, since an imogolite yields superior water vapor adsorption performance, its application as a heat pump heat exchange material, a dew condensation prevention agent, an autonomous humidity control material and the like is also expected.
In particular, since desiccant air conditioning aims to eliminate the moisture content in the air that is introduced from the outside air, it is required to efficiently eliminate the moisture content even from the high humidity air in the summer, and the adsorbent that is sought in the desiccant air conditioning is generally demanded of a high adsorption amount at a relative humidity of roughly 5% to 60%.
Under the foregoing circumstances, the mass synthesis at an industrially low cost is demanded without losing the foregoing characteristics of the tubular aluminum silicate. However, since a protoimogolite has a short tube length, it is unable to form fine pores of tubes as with an imogolite, and it can hardly adsorb water vapor in a medium-humidity range or a high-humidity range. Because of these factors, it is necessary to synthesize substances that can adsorb water vapor even in a medium-humidity range or a high-humidity range.
Moreover, even with an imogolite, since it has a relatively long tube length in which the aspect ratio is approximately 1000, the substance shows good performance in a low-humidity range and a high-humidity range at a relative humidity of 90% or higher, but has a drawback in that the adsorption amount is small in a medium-humidity range.
Conventionally, the development of adsorbents utilizing the unique fine a pore of an imogolite or an amorphous imogolite has been made (refer to Patent Documents 1 and 2). Nevertheless, with these conventional methods, it was not possible to achieve an adsorption amount of 30 wt % at a relative humidity of 60%.    [Patent Document 1] Japanese Patent Laid-Open Publication No. 2001-064010    [Patent Document 2] Japanese Patent Laid-Open Publication No. 2004-059330